A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied from an external reactant supply source. Fuel cells operate by converting a reactant fuel such as hydrogen or a hydrocarbon (e.g., methanol) to electrical power through an electrochemical process rather than by combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as proper reactants (i.e., a fuel and an oxidant) are supplied from an outside source.
As is appreciated by those skilled in the art, most conventional micro-scale fuel cell systems include a stack of electrically interconnected electrode pair assemblies (commonly referred to as a fuel cell stack assembly), wherein each electrode pair is configured to receive and react with selected reactants (e.g., methanol and air flowstreams delivered across respective outer electrode surfaces). The interposing electrolyte of most conventional micro-scale liquid-air fuel cell systems (e.g., direct methanol fuel cell (DMFC) systems) generally consist of a solid polymer proton exchange membrane (PEM) (e.g., NAFION). Electrode pair assemblies that include solid polymer proton exchange membranes (sometimes referred to as solid polymer electrolytes (SPEs)) are known as membrane electrode assemblies (MEAs). Exemplary in this regard are the MEAs and micro-scale methanol-air fuel cell systems disclosed in U.S. Pat. No. 5,364,711 to Yamada et al., U.S. Pat. No. 5,523,177 to Kosek et al., U.S. Pat. No. 5,559,638 to Aoki et al., U.S. Pat. No. 5,773,162 to Surampudi et al., U.S. Pat. No. 5,874,182 to Wilkerson et al., U.S. Pat. No. 5,945,231 to Narayanan et al., U.S. Pat. No. 5,992,008 to Kindler, and U.S. Pat. No. 6,387,559 to Koripella. These known micro-scale fuel cell systems all comprise an interconnected series of electrode pair assemblies, wherein each electrode pair utilizes a solid polymer proton exchange membrane as a separator and as a proton (H+) transfer medium. As is appreciated by those skilled in the art, a solid polymer proton exchange membrane refers to a perfluorosulfonic acid membrane such as, for example, NAFION (E.I. Du Pont de Nemours and Company, U.S.A.). In general, many of these ionomer membranes comprise a polytetrafluoroethylene (PTFE) polymer backbone chain that is several units (e.g., n=6-10) in length, with a flexible perfluorinated vinyl polyether (m≧1) pendant branch, wherein the pendant branch has a terminal acidic (sulfonic) group to provide for cation (proton) exchange capability. As an example, such an ionomer unit may have the following structure (equivalent weight of about 1,200):

There are, however, significant technical problems associated with known micro-scale PEM-based methanol-air fuel cell systems. More specifically, and because these type of systems employ a central PEM, conventional micro-scale methanol-air fuel cell systems suffer from several drawbacks including: (1) each electrode possesses only a limited two-dimensional catalyzed reaction zone, wherein each zone is defined by the interface between the reactant flowstream and one of the catalyzed outer surfaces of the central PEM, (2) the central PEM tends to degrade rapidly over time if not sufficiently and continuously hydrated, and (3) unreacted fuel is able to “cross-over” from the anode compartment to the cathode compartment via the central PEM and degrade the cathode-side catalyst. These drawbacks are generally inherent to most all types of conventional PEM-based methanol-air fuel cell systems even though numerous solutions have been contemplated.
In contrast, certain liquid-liquid fuel cell systems avoid many of the drawbacks associated with PEM-based methanol-air fuel cell systems because these types of systems do not utilize a central PEM. In general, liquid-liquid fuel cell systems typically comprise electrode pairs and related stack assemblies that include a series of microfluidic flow channels for flowing liquid reactant/electrolyte flowstreams (i.e., electrolytic fuel and oxidant flowstreams referred to herein as anolyte and catholyte flowstreams, respectively) adjacent to and/or through discrete regions of accompanying porous electrode structures. Exemplary in this regard are the liquid-liquid fuel cell systems disclosed in U.S. Pat. No. 3,261,717 to Shropshire et al., U.S. Pat. No. 3,281,274 to Moerikofer, U.S. Pat. No. 3,318,735 to Tammy et al., U.S. Pat. No. 3,350,227 to Moerikofer et al., U.S. Pat. No. 3,979,225 to Smith et al., U.S. Pat. No. 4,528,250 to Struthers, and U.S. Pat. No. 6,713,206 to Markoski et al. Most all of these known fuel cell systems use nitric acid as the primary oxidant (wherein the nitric acid may be continuously regenerated by intimate exposure to oxygen gas supplied from the air) and methanol or other like hydrocarbon as the fuel. These known fuel cell systems, however, have significant technical limitations and are not optimal for generating power within a closed recirculating microfluidic liquid-liquid feed system adapted for use with a portable electronic device.
Accordingly, there is still a need in the art for new types of microscale liquid-liquid fuel cell systems. The present invention fulfills these needs and provides for further related advantages.